1. Field of the Invention
The present invention relates to a hydraulic driving circuit in which a variable displacement hydraulic pump and a variable displacement hydraulic motor are connected in a closed circuit. More particularly, the present invention pertains to a structure which is designed such that control of travel torque during the combined operation consisting of travel and loader operations is performed by controlling the displacement of the variable displacement hydraulic motor in accordance with a load for a loader operation.
2. Description of the Related Art
Construction machines, such as a wheel loader, including a first hydraulic circuit for travelling the machine and a second hydraulic circuit for a loader attachment used for, for example, digging, are known. In such construction machines, how power is distributed for travelling and loader attachment is an essential matter. If the moving force is set too high with regard to soil pushing and tracting, the tractive force may become too large with respect to the loading force. This may cause the tires to slip when a bucket pushed into the soil is lifted, reducing the tractive force during the combined operation. Various types of circuits have been proposed to solve this problem.
FIG. 6 shows an example of the above-described type of conventional hydraulic driving circuit which has been proposed by the present inventors. In FIG. 6, a reference numeral HC1 denotes a hydraulic circuit for travelling, and HC2 denotes a hydraulic circuit for a loader operation. A variable displacement hydraulic pump 2 for travelling, a charge pump 3 and a hydraulic pump 4 for a loader operation are rotated by an engine 1, such as a Diesel engine. When a forward/reverse change-over valve 6 is set to neutral, the hydraulic oil discharged from the charge pump 3 passes through a restrictor 5 and the forward/reverse change-over valve 6 and is then led to both cylinder chambers 8a and 8b of a cylinder 8 via conduits 7A and 7B to equalize the pressure in both cylinder chambers 8a and 8b. A piston 8c is located at a neutral position, and the displacement (hereinafter also referred to as a tilting amount) of the variable displacement hydraulic pump 2 is thus zero, thereby making the discharge thereof zero.
When the forward/reverse change-over valve 6 is operated to the "1" side by an operation lever 10, the upstream pressure of the restrictor 5 is applied to the cylinder chamber 8a while the downstream pressure is applied to the cylinder chamber 8b, thus displacing the piston 8c to the right by a distance corresponding to a differential pressure between the upstream and downstream of the restrictor 5. The tilting amount of the variable displacement hydraulic pump 2 is set, and the variable displacement hydraulic pump 2 thus discharges the hydraulic oil into a main conduit 11A at a rate determined in accordance with the tilting amount thereof, rotating the variable displacement hydraulic motor 12 and thereby moving the vehicle forward. When the forward/reverse change-over valve 6 is operated to the "m" side, tilting of the variable displacement hydraulic pump 2 is set in a reverse direction. The hydraulic oil is discharged into a main conduit 11B to rotate the hydraulic motor 12 in a reverse direction.
The speed of the engine 1 is adjusted using an accelerator pedal 13. The discharge rate of the charge pump 3 is proportional to the engine speed, and the difference in the upstream and downstream pressures of the restrictor 5 increases as the engine speed increases. Therefore, the tilting amount of the variable displacement hydraulic pump 2 increases as the engine speed increases.
The driver moves the vehicle forward by pressing the accelerator pedal 13, and then forces the front portion of the vehicle which is not shown (which may be a bucket or the like and is referred to as a loader attachment) into a stack of pebbles or the like. When the bucket packed with pebbles or the like is lifted by operating a loader attachment operation control valve (not shown) to scoop up the pebbles or the like, the hydraulic pump 4 of the loader attachment is loaded. When the sum of the loads applied to the hydraulic pumps 2 and 4 exceeds the engine output, the engine speed decreases. The difference in the upstream and downstream pressures of the restrictor 5 decreases, moving the piston 8c of the cylinder 8 toward the neutral side and thereby decreasing the tilting amount of the variable displacement hydraulic pump 2. As a result, stalling of the engine is avoided, and the engine continues rotating at a speed suited to the sum of the travel and loader operation loads.
Both pressure in the circuit for travelling (pressure from the load for travelling) Pt and pressure in the circuit for a loader attachment (pressure from loader operation load) Pf act on a switching valve 25. When (Pt+Pf) exceeds the pressure Pr set by a spring 25a, the switching valve 25 is opened, causing the conduits 7A and 7B to communicate with each other through conduits 26A and 26B. The pressures in the cylinder chambers 8a and 8b of the cylinder 8 are equalized, and the tilting amount of the variable displacement hydraulic pump 2 thus starts decreasing toward the neutral position, i.e., toward zero. As a result, the pressure in the circuit for travelling Pt decreases, and the pressure in the conduit 11C for pushing switching valve 25 thus decreases. When (Pt+Pf) .ltoreq.Pr, the switching valve 25 is switched over to the closed position, and the displacement of the variable displacement hydraulic pump 2 is thus increased, whereby increasing the discharge rate of the variable displacement hydraulic pump 2 and hence the pressure in the circuit for travelling. When (Pt+Pf) &gt;Pr again, the switching valve 25 is switched over to the open position, and the discharge rate of the variable displacement hydraulic pump 2 thereby decreases. By repeating the above-described operation, the travelling circuit pressure is controlled to a predetermined value to control the output torque of the hydraulic motor 12.
The above-described operation is based on the relationship between (Pt+Pf) and Pr. The relationship between the pressure in the circuit for the loader attachment and the pressure in the circuit for travelling is shown in FIG. 7. In FIG. 7, Ptmax denotes the maximum pressure in the circuit for travelling, Ps denotes the circuit pressure when the tire is slipping, Pfmax and Pfmin respectively denote the maximum pressure in the circuit for the loader attachment and the pressure when the circuit for loader attachment is not operated, Ptlow denotes the pressure in the circuit for travelling when the pressure in the circuit for the loader attachment is Pfmax. When the loader attachment is not operated, the pressure in the circuit for travelling tends to rise up to Ptmax. However, slipping of the tire occurs when the pressure in the circuit for travelling reaches a certain value. When the loader attachment is operated when the tire is slipping, the pressure in the circuit for loader attachment Pf rises while the pressure in the circuit for travelling Pt drops in accordance with the graph of FIG. 7. When the pressure in the circuit for the loader attachment is Pfs or above, the pressure in the circuit for travelling is reduced to Ps or below and the tire stops slipping.
FIG. 8 shows the P - q diagram for the hydraulic pump 2. The cut-off pressure Pc is dependent on the pressure in the circuit for the loader attachment Pf. At Pfmin, the cut-off pressure is Pcmax (=Ptmax: FIG. 7). At Pfmax, the cut off pressure is Pcmin (=Ptlow: FIG. 7). The P - q diagram is set between Max and Min in accordance with the pressure in the circuit for the loader attachment.
The tilting amount of the hydraulic motor 12 is controlled in the manner described below.
A higher pressure selecting valve 21 selects a higher one among the pressure in the conduits 11A and 11B and the selected pressure is introduced to the conduit 11C. The pressure in the conduit 11C is led to the small-diameter chamber of a tilting cylinder 22 for the hydraulic motor and to a pilot port 23a of a change-over valve 23. When the pressure in the circuit for travelling is lower than the pressure Pset set by a spring 23b of the change-over valve 23, the change-over valve 23 is located at "a" position, causing the large-diameter chamber of the tilting cylinder 22 to communicate with a tank 24. A piston 22a is moved to the left by the pressure in the circuit for travelling led into the small-diameter chamber. Thus, the tilting amount of the hydraulic motor 12 is maintain at the minimum value. When the pressure in the circuit for travelling in the conduit 11C increases to the pressure Pset set by the spring 23b or above, the change-over valve 23 is switched over to "b" position. The pressure in the circuit for travelling is also led into the large-diameter chamber of the tilting cylinder 22, and the piston 22a thereby moves to the right due to the difference in the area between the large- and small-diameter chambers. Rightward movement of the piston 22a increases the tilting amount of the hydraulic motor 12. The tilting amount of the hydraulic motor 12 is balanced to a given value between qmin and qmax shown in FIG. 9 in accordance with the load for travelling.
In FIG. 6, a reference numeral 14 denotes a cross overload relief valve; 15, a flushing valve; 16A and 16B, checking valves which are connected to the charge pump 3 through conduits 17a and 17b, and 18, a relief valve for the charging pump.
In the above-described conventional hydraulic driving circuit, when the sum of the loads for travelling and loader operation exceeds the output of the engine and the engine speed is thereby reduced, the tilting amount of the variable displacement hydraulic pump 2 is reduced in accordance with the engine speed, and the load is thereby restricted. Furthermore, torque for travelling is reduced by reducing the displacement of the variable displacement hydraulic pump as the pressure in the circuit for the loader attachment is greater.
However, the conventional hydraulic driving circuit has the following drawbacks.
(1) When the pressure in the circuit for the loader attachment exceeds a predetermined value, the tilting amount of the variable displacement hydraulic pump 2 is reduced using the cut-off control to reduce the pressure in the circuit for travelling and thereby limit the output torque of the hydraulic motor 12. Therefore, if the motor load is inertial, a slight change in the tilting amount rapidly and greatly changes the pressure. This makes stable control of the circuit pressure difficult. Furthermore, since the load changes greatly during the digging operation, it acts as a disturbance, making stable control of the circuit pressure more difficult. PA1 (2) As the pressure which acts on two pilot pressure receiving portions 25b and 25c of the switching valve 25 is high, even when the spring 25a has a large size, each diameter of the pistons of the switching valve 25 must be made very small (generally, about 2 mm). And since the area ratio between the two pistons affects to reduce the output torque, inaccurate dimension of the pressure receiving portions varies the reduction ratio of the output torque. This results in variations in the distribution of power to the travelling and loading forces during the combined operation for each machine.